I believe there was a mention in the Day 1 BUILD keynote somewhere that Samsung's work on .NET on ARM (built for Tizen specifically, but I would imagine generically useful) was going to be open sourced to the .NET Core project at some point.
Also, the Win32 x86 emulation I would imagine would support most/all .NET Win32 applications, to whatever extent the Win32 .NET Framework gets added to emulation image.